1. Field
The present disclosure relates to casting of prefabricated concrete products with a substantially horizontal slipform casting process, where the concrete mix is fed at least in one step though a limited cross-section moving progressively along with the cast.
2. Description of Related Art
Several different slipform casting methods and devices are known in the art. The two main slipform casting methods for casting concrete products are extruder and slipformer methods. In the extruder method concrete mix is fed in a single feeding stage from a concrete mass container to feed screws which feed screws extrude the concrete mix to a slipform casting mold defined by upper surface of a casting bed and side and top plates of a casting machine. When casting hollow core slabs, the feed screws are followed by core forming mandrels forming the cores in form of longitudinal voids in the concrete product to cast. The compacting of the concrete product to be cast is achieved by vibrating and/or leveling motion of the side and top plates, and the forming of the cores is secured by back-and-forth compacting motion of an entity formed of the feed screw and the attached core mandrel. The casting machine moves along the casting bed driven by reaction force from the feed screws extruding the concrete mass and optionally with an additional drive motor. The ready cast product remains on the casting bed as the casting progresses.
In the slipformer casting method concrete mix is fed in at least two feeding stages from a concrete mass container to a slipform casting mold. In the first feeding stage concrete mass is fed to a lower portion of the casting mold formed by a top surface of a casting bed and side plates of a casting machine. The first feed stage of concrete mix is followed by vibrating shoes and core forming mandrels that by vibrating the concrete mix compacts the cast concrete mix and form the final shape of the lower part of the concrete product to be cast. In the second feeding stage concrete mix is fed onto the end portions of the core forming mandrels and on the previously cast concrete mix for casting the upper portion of the product to be cast, after which the concrete mix is compacted with a vibrating plate defining the upper surface of the slipform casting mold and located at the rear part of the casting machine. The ready cast product remains on the casting bed as the casting progresses.
Slipform casting is generally used for casting long products with uniform cross-section, such as massive or hollow core slabs, which are cut to predefined lengths after the concrete is cured.
Patent publication EP 1 843 882 B1 discloses a slipform casting method and apparatus for casting concrete products with cores, where the thickness of the cast concrete product can be changed by changing the height of the core forming mandrel and the location of the top plate of the slipform casting mold during the casting process. The disclosed apparatus may also be used to cast products with different thicknesses.
In concrete hollow core slabs thickness of the webs between the cores is one of the main features defining a shear stress capacity for the slab, especially in the end areas of the slab, which end areas are generally used for providing support for the slab in buildings.
The thickness of the webs between the cores is thus dimensioned based on the maximum shear stresses affecting the end areas of the slab. These maximum shear stresses do not affect the central area of the slab, but since the core forming members forming the hollow cores and the webs in the slab cannot be changed in slipform casting machines during the casting process, the webs have uniform thickness throughout the cast slab. This causes increased weight and decreased load capacity, for example, for the slabs.
One known solution for increasing the strength of the end areas of a hollow core slab is to fill the ends of the cores with concrete mass after the slipform casting process and thus add concrete in the critical areas of the slab. This however increases the use of concrete mass and slows down the manufacturing process.